A SRS is a signal used for measuring Channel State Information (CSI) between a User Equipment (UE) and an e-Node-B (eNB). In a Long Term Evolution (LTE) system, the UE periodically transmits an uplink SRS according to parameters indicated by the eNB, such as a bandwidth, a frequency domain location, a sequence cyclic shift, a period and a sub-frame offset etc. The eNB determines the uplink CSI of the UE according to the received SRS, and performs operations such as frequency domain selective scheduling, and closed loop power control etc. according to the obtained CSI.
In the LTE system, the SRS sequence transmitted by the UE is obtained by performing cyclic shift α on a root sequence rN,V(n) in the time domain. Different SRS sequences can be obtained by performing different cyclic shifts α on the same root sequence, and the obtained SRS sequences are orthogonal with each other. Therefore, these SRS sequences can be allocated to different UEs for use, so as to implement code division multiple access among the UEs. In the LTE system, the SRS sequence defines 8 cyclic shifts α, which are given by the following equation:
  α  =      2    ⁢    π    ⁢                  n        SRS        cs            8      
wherein nSRScs is indicated by signaling with 3 bits, which are 0, 1, 2, 3, 4, 5, 6 and 7 respectively. That is to say that the UE in the cell has 8 available code resources in the same time-frequency resource, and the eNB can configure at most 8 UEs to transmit the SRS simultaneously on the same time-frequency resource. The above equation can be considered as equally dividing the SRS sequence into 8 portions in the time domain; however, as a length of the SRS sequence is a multiple of 12, the minimum length of the SRS sequence is 24.
In the LTE system, the frequency domain bandwidth of the SRS is configured by a tree structure. Each kind of SRS bandwidth configuration corresponds to one tree structure, and the SRS bandwidth of the highest layer corresponds to the largest SRS bandwidth of the SRS bandwidth configuration, or is called as a SRS bandwidth range. Tables 1 to 4 provide the SRS bandwidth configuration in different uplink SRS bandwidth ranges, wherein NRBUL is the number of Resource Blocks (RBs) corresponding to the uplink SRS bandwidth.
TABLE 1SRS bandwidth configuration of 6 ≦ NRBUL ≦ 40SRS-SRS-SRS-SRS-SRS bandwidthBandwidthBandwidthBandwidthBandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb03611234341132116282422241464141320145414141614441415121434141681424141741414141
TABLE 2SRS bandwidth configuration of 40 < NRBUL ≦ 60SRS-SRS-SRS-SRS-SRS bandwidthBandwidthBandwidthBandwidthBandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb04812421224314811638242240120245413361123434143211628242524146414162014541417161444141
TABLE 3SRS bandwidth configuration of 60 < NRBUL ≦ 80SRS-SRS-SRS-SRS-SRS bandwidthBandwidthBandwidthBandwidthBandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb0721243122431641322162442601203454134812421224344811638242540120245416361123434173211628242
TABLE 4SRS bandwidth configuration of 80 < NRBUL ≦ 110SRS-SRS-SRS-SRS-SRS bandwidthBandwidthBandwidthBandwidthBandwidthconfigurationBSRS = 0BSRS = 1BSRS = 2BSRS = 3CSRSmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb0961482242461961323162442801402202453721243122434641322162445601203424164812421224374811638242
The tree structure of the SRS bandwidth will be illustrated taking SRS bandwidth configuration index 1, i.e., CSRS=1 in table 1. BSRS=0 is layer 0, and is the highest layer of the tree structure. The SRS bandwidth of this layer is the bandwidth corresponding to 32 RBs, and is the largest SRS bandwidth of the SRS bandwidth configuration 1; BSRS=1 is layer 1, the SRS bandwidth of this layer is the bandwidth corresponding to 16 RBs, and the SRS bandwidth of the upper layer, i.e., layer 0 is divided into 2 SRS bandwidths of layer 1; BSRS=2 is layer 2, the SRS bandwidth of this layer is the bandwidth corresponding to 8 RBs, and the SRS bandwidth of the upper layer, i.e., layer 1 is divided into 2 SRS bandwidths of layer 2; and BSRS=3 is layer 3, the SRS bandwidth of this layer is the bandwidth corresponding to 4 RBs, and the SRS bandwidth of the upper layer, i.e., layer 2 is divided into 2 SRS bandwidths of layer 3, and the tree structure thereof is shown in FIG. 1.
In the LTE system, the eNB firstly allocates a SRS bandwidth configuration index CSRS to all UEs in a cell, and the UE can determine to use which table in tables 1 to 4 according to CSRS and a number of RBs corresponding to the current uplink bandwidth, i.e., NRBUL, and then can determine the SRS bandwidth configuration that is used by the current cell according to CSRS. For a plurality of UEs, the eNB can further allocate one SRS bandwidth index BSRS (or is called as an index of a located layer) to the UE. The UE can obtain the SRS bandwidth used by the UE according to the SRS bandwidth configuration and the SRS bandwidth index BSRS in the cell. For example, if the SRS bandwidth configuration index of the current cell CSRS=1 and NRBUL=50, the UE determines that the SRS bandwidth configuration of the current cell is the second row in table 2. If the SRS bandwidth index allocated by the eNB of the current cell to the UE is 1, the SRS bandwidth of the UE occupies 16 RBs, and the SRS bandwidth location of the UE is within the SRS bandwidth range, i.e., within a range of the largest SRS bandwidth with 48 RBs.
The UE will determine an frequency domain initial location for transmitting the SRS by itself according to a frequency domain location nRRC of the upper layer signaling transmitted by the eNB after obtaining the own SRS bandwidth. As shown in FIG. 2, the UEs to which different nRRC are allocated will transmit the SRS in different regions of the SRS bandwidth of the cell.
The sequence that is used by the SRS is selected from a group of Demodulation Reference Signal (DMRS) sequences. When the SRS bandwidth of the UE is 4 RBs, a Computer Generated (CG) sequence with a length of 2 RBs will be used; and when the SRS bandwidth of the UE is larger than 4 RBs, a Zadoff-Chu sequence with a corresponding length will be used.
In addition, in the same SRS bandwidth, the sub-carriers of the SRS are placed at intervals, that is, the SRS is transmitted using a comb structure, and the number of frequency combs in the LTE system is 2, which also corresponds to the time domain RePetition Factor (RPF) 2. As shown in FIG. 3, when each UE transmits the SRS, only one of the two frequency combs is used, i.e., comb=0 or comb=1. Thus, the UE only uses a sub-carrier with an even or odd frequency domain index to transmit the SRS according to the indication of 1 bit upper layer signaling. Such a comb structure allows more UEs to transmit the SRS in the same SRS bandwidth.
In the same SRS bandwidth, a plurality of UEs can use different cyclic shifts on the same frequency comb, and then transmit the SRS by code division multiplexing, or two UEs can transmit the SRS by frequency division multiplexing on different frequency combs. For example, in the LTE, there are 8 available cyclic shifts for the UE who transmits the SRS in a certain SRS bandwidth (4 RBs), and there are 2 available frequency combs. Therefore, the UE has 16 resources which can be used to transmit the SRS in all, that is, at most 16 SRSs can be transmitted simultaneously in the SRS bandwidth. As the uplink Single User Multiple Input Multiple Output (SU-MIMO) is not supported in the LTE system, there is only one antenna for the UE to transmit the SRS at each time, and thus, one UE only needs one SRS resource. Therefore, in the above SRS bandwidth, the system can at most multiplex 16 UEs simultaneously.
A LTE-Advanced (LTE-A) system is the next generation evolution system of the LTE system. The SU-MIMO is supported in the uplink, and at most 4 antennas can be used as uplink transmitting antennas. That is, the UE can transmit the SRS on a plurality of antennas simultaneously at one time, while the eNB needs to estimate a state on each channel according to the SRS received by each antenna.
The current research on the LTE-A proposes that a non-precoded (that is, antenna-specific) SRS should be used in uplink communication. At this point, when the UE uses a plurality of antennas to transmit the non-precoded SRS, the SRS resources required by each UE will increase, which leads to the number of the UEs which can be multiplexed simultaneously in the system reducing.
For example, in a certain SRS bandwidth (4 RBs), if each UE uses 4 antennas to transmit the SRS, the number of the resources required by each HE is 4. Upon the above description that the number of the SRS resources which can be supported in one SRS bandwidth is 16 in all, the number of UEs which can be multiplexed in the SRS bandwidth is reduced to 4. The number of users who can be multiplexed in the system simultaneously is ¼ of that of the original LTE.
Since the requirement of the LTE-A proposes that the number of the users who can be accommodated in the LTE-A system should be not less than that in the LTE system, the requirement is contradiction with the above practice that the number of the users reduces when the SRS is transmitted by a plurality of antennas. Solving the contradiction between the requirements on the user capacity of the LTE-A and the reduction of the number of the users when the SRS is transmitted by a plurality of antennas can be implemented by increasing available SRS resources in the system in practical. Therefore, how to increase the SRS resources in the system becomes a problem to be solved.